Abstract. The present analysis is concerning the criteria for the onset of flow reversal of the fully developed flow mixed convection of nanofluids in a vertical channel filled with porous medium. The governing equations and the critical values of the buoyancy force are solved and calculated numerically via package in MAPLE. It was found that the critical values of mixed convection parameter for the occurrence of reversed flow decreases with increasing temperature difference ratio and increases with increasing nanoparticles mass flux.